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Subject[PATCH 09/22] mm/hmm: Simplify hmm_get_or_create and make it reliable
Date
From: Jason Gunthorpe <jgg@mellanox.com>

As coded this function can false-fail in various racy situations. Make it
reliable and simpler by running under the write side of the mmap_sem and
avoiding the false-failing compare/exchange pattern. Due to the mmap_sem
this no longer has to avoid racing with a 2nd parallel
hmm_get_or_create().

Unfortunately this still has to use the page_table_lock as the
non-sleeping lock protecting mm->hmm, since the contexts where we free the
hmm are incompatible with mmap_sem.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Philip Yang <Philip.Yang@amd.com>
---
mm/hmm.c | 77 ++++++++++++++++++++++----------------------------------
1 file changed, 30 insertions(+), 47 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 080b17a2e87e..0423f4ca3a7e 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -31,16 +31,6 @@
#if IS_ENABLED(CONFIG_HMM_MIRROR)
static const struct mmu_notifier_ops hmm_mmu_notifier_ops;

-static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
-{
- struct hmm *hmm = READ_ONCE(mm->hmm);
-
- if (hmm && kref_get_unless_zero(&hmm->kref))
- return hmm;
-
- return NULL;
-}
-
/**
* hmm_get_or_create - register HMM against an mm (HMM internal)
*
@@ -55,11 +45,16 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
*/
static struct hmm *hmm_get_or_create(struct mm_struct *mm)
{
- struct hmm *hmm = mm_get_hmm(mm);
- bool cleanup = false;
+ struct hmm *hmm;
+
+ lockdep_assert_held_exclusive(&mm->mmap_sem);

- if (hmm)
- return hmm;
+ /* Abuse the page_table_lock to also protect mm->hmm. */
+ spin_lock(&mm->page_table_lock);
+ hmm = mm->hmm;
+ if (mm->hmm && kref_get_unless_zero(&mm->hmm->kref))
+ goto out_unlock;
+ spin_unlock(&mm->page_table_lock);

hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
if (!hmm)
@@ -74,57 +69,45 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
hmm->notifiers = 0;
hmm->dead = false;
hmm->mm = mm;
- mmgrab(hmm->mm);

- spin_lock(&mm->page_table_lock);
- if (!mm->hmm)
- mm->hmm = hmm;
- else
- cleanup = true;
- spin_unlock(&mm->page_table_lock);
+ hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
+ if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
+ kfree(hmm);
+ return NULL;
+ }

- if (cleanup)
- goto error;
+ mmgrab(hmm->mm);

/*
- * We should only get here if hold the mmap_sem in write mode ie on
- * registration of first mirror through hmm_mirror_register()
+ * We hold the exclusive mmap_sem here so we know that mm->hmm is
+ * still NULL or 0 kref, and is safe to update.
*/
- hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
- if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
- goto error_mm;
-
- return hmm;
-
-error_mm:
spin_lock(&mm->page_table_lock);
- if (mm->hmm == hmm)
- mm->hmm = NULL;
+ mm->hmm = hmm;
+
+out_unlock:
spin_unlock(&mm->page_table_lock);
-error:
- mmdrop(hmm->mm);
- kfree(hmm);
- return NULL;
+ return hmm;
}

static void hmm_free_rcu(struct rcu_head *rcu)
{
- kfree(container_of(rcu, struct hmm, rcu));
+ struct hmm *hmm = container_of(rcu, struct hmm, rcu);
+
+ mmdrop(hmm->mm);
+ kfree(hmm);
}

static void hmm_free(struct kref *kref)
{
struct hmm *hmm = container_of(kref, struct hmm, kref);
- struct mm_struct *mm = hmm->mm;

- mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
+ spin_lock(&hmm->mm->page_table_lock);
+ if (hmm->mm->hmm == hmm)
+ hmm->mm->hmm = NULL;
+ spin_unlock(&hmm->mm->page_table_lock);

- spin_lock(&mm->page_table_lock);
- if (mm->hmm == hmm)
- mm->hmm = NULL;
- spin_unlock(&mm->page_table_lock);
-
- mmdrop(hmm->mm);
+ mmu_notifier_unregister_no_release(&hmm->mmu_notifier, hmm->mm);
mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
}

--
2.20.1
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